The objective of this proposal is to improve our understanding of the role that environmental tobacco smoke (ETS) plays in asthma. Asthma, a disease characterized by increased airway reactivity and inflammation in response to a variety of stimuli, is emerging as the most prevalent and serious environmental health problem among children in the United States. Numerous studies, both prospective and cross-sectional, suggest that exposure to ETS is one of the predominate risk factors for childhood asthma, but this has not been confirmed in a controlled trial. This proposed study will be the first large scale, controlled trial utilizing passive controls designed to test the effects of reducing indoor ETS on asthma symptoms, pulmonary function, airway inflammation (as measured with expired nitric oxide), and health services utilization. We request 48 months of support to conduct a randomized, double-blind prospective trial involving 240 children with doctor-diagnosed asthma who are exposed to environmental tobacco smoke, to test the efficacy of reducing such exposure on asthma symptoms. The intervention consists of placement of 2 high efficiency air filtration with activated carbon, potassium permanganate and zeolite filter insert (HEPA-CPZ) to reduce exposure to ETS in the experimental homes and inactive (placebo) units in the control group homes. We will test the following hypotheses: (1.0) Children assigned to the ETS reduction group will have significant improvements in asthma symptoms and pulmonary function compared with children in the control group. (1.1) Children assigned to the ETS reduction group will have >25 percent reduction in exacerbations of asthma during one year of follow-up compared with those in the control group. (1.2) Children assigned to the ETS reduction group will have improvement in pulmonary function as measured by >10 percent difference in forced expiratory volume at 1 second (FEV 1) and >15 percent difference in forced expiratory fraction of 25 percent-75 percent (FEF25-75 percent) at 12-month follow-up compared with those in the control group. (1.3) Children assigned to the ETS reduction group will have > 10 percent reduction in exhaled nitric oxide, a measure of airway inflammation during one year of follow-up compared with the control group. (1.4) Children assigned to the ETS reduction group will have a >20 percent reduction in health services utilization for asthma during one-year follow-up compared with those in the control group. (2.0) Children who are exposed to higher levels of ETS, as measured by cotinine, will experience higher rates of asthma symptoms and diminished pulmonary function in a dose-response relationship. Our proposed randomized, controlled trial, consisting of biologic markers of exposure, will provide evidence of a causal pathway between the exposure to environmental tobacco smoke and asthma, substantially improve our understanding of the contribution of environmental tobacco smoke to childhood asthma, and provide important information to develop prevention strategies.